Infinitely variable hydrostatic-mechanical power shift gearboxes are advantageously suitable for motor vehicles since they can be continuously adjusted within wide ranges and, in addition, have good efficiencies. The relatively wide range of adjustment allows the internal combustion engine to be operated on preferred characteristics. Such characteristics can be, for example, the curve for minimum fuel consumption or a line for good acceleration behavior.
The present invention represents a further development of the gearboxes according to U.S. Pat. Nos. 3,580,107, 3,709,060, 3,888,139 and DE 31 47 447 C2.
FIGS. 1, 17, 18 and 20 of U.S. Pat. No. 3,580,107 discloses that the maximum hydrostatic power flow occurs at half the maximum speed of the displacement-type machines, at half the maximum displacement volume and at 0.8-times the hydrostatic pressure. The characteristic hydrostatic power resulting from these values thus is five times the effective maximum hydrostatic power, necessitating displacement-type machines of relatively large construction. The concept results in each auxiliary transmission stage having a different transmission ratio.
The infinitely variable hydrostatic-mechanical power shift gearbox according to U.S. Pat. No. 3,709,060 starts purely hydrostatically. Apart from this starting gear, it only has two other gears. As a result, the displacement-type machines are of relatively large construction. The concept results in the two auxiliary transmission stages having different transmission ratios.
The gearbox according to U.S. Pat. No. 3,888,139 has four gears with two planetary stages as auxiliary transmission stages. Here, too, starting is performed purely hydrostatically. During this process, a reactive power circulates in the gearbox which amounts to 50% of the throughout power. Overall, the concept requires a relatively large number of wheels and gear-changing clutches, which means a great expenditure.
German Patent Specification 31 47 447 C2 describes a gearbox in which starting is performed by means of a conventional friction clutch. this is why relatively small displacement-type machines are sufficient. The auxiliary transmission stages constructed as spur gear stages have the same transmission ration in each case for two adjacent gears. Toothed-wheel clutches are provided for the gear changes.
In the gearboxes according to the Patent Specifications mentioned, gear changing actions for a gear change are to occur at synchronous speeds. During a gear change, however, the power flow direction reverses in the hydrostatic transmission. during the process, the displacement-type machines exchange their functions as pump and motor. If a displacement-type machine operates as a pump in the old gear, its displacement volume must be adjusted to a greater value, for covering the leakage oil flows and for achieving synchronous speeds, than that of the theoretical displacement volume which would be present for leakage-oil-free operation. After the gear change has been effected, the displacement-type machine then acts as motor. In the near gear, its adjusted displacement volume is too great. It should now be less than its theoretical volume. as a result, the synchronous running of the transmission parts affected by the gear change is temporarily disturbed. A stress with a pressure rise arises even before the old gear is disengaged. The gear-changing force provided suddenly separates the old gear which is thus still loaded, resulting in a gear-changing jolt. If the displacement-type machine is running as a motor in the old gear, its displacement volume is too small in the near gear where it acts as a pump. This also causes a jolt.
Engaging a new gear at synchronous speeds can be impeded by the fact that tooth is facing tooth in the gear-changing clutch. Due to dynamically caused overshooting of the volume-controlled displacement-type machine, synchronous running can be disturbed so that the new gear is not engaged.
The gear-changing forces for actuating the gear-changing clutches must be dimensioned in such a manner that immediately after the load is displaced from the old to the new gear, which is effected by a corresponding change in displacement volume, the old gear is disengaged free of torque. If the gear-changing force is too great, the old gear would be prematurely pulled out. This would be associated with jolting and wear. If the gear-changing force is too small, it would not be adequate for separating the gear-changing clutch. The gear-changing forces are provided by gear-changing cylinders which are supplied by the feed circuit of the hydrostatic transmission. The feed pressure depends on the operating conditions. However, a fluctuating feed pressure also results in variable gear-changing forces and thus disturbs jolt-free gear changing.
Multiple-disk gear-changing clutches which are provided in the figures of U.S. Pat. Nos. 3,709,060 and 3,888,139 can compensate differential speeds by slipping and can be changed in overlapping manner for high gear-changing quality. However, multiple-disk gear-changing clutches are much larger compared with toothed-wheel gear-changing clutches and generate idling power losses in the opened condition which decrease the efficiency of the transmission.
However,the use of toothed-wheel gear-changing clutches requires particular control and constructional measures to achieve good gear-changing qualities. U.S. Pat. No. 3,580,107 and DE 3 147 447 C7 do not provide any indications as to how the disturbance effects described above can be eliminated.
The gearbox according to German Patent Specification 3 147 447 C2 comes closest to the present invention. It has the following objects:
shorter constructional length so that it can also be installed in vehicles having a transverse engine PA1 reduction in constructional expenditure PA1 continuously variable reverse of the vehicle and utilization of the gears provided both for forward and for reverse travelling as can be required in the case of construction machines and tractors PA1 improvement in the gear-changing quality, that is to say avoidance of gear-changing jolts.